- Conference date: 29 September–4 October 2009
- Location: Rhodes, Greece
A formulation of Koopmans' theorem is derived for high-spin half-filled open shells in the restricted openshell Hartree-Fock (ROHF) method based on a variational treatment of both the initial (non-ionized) open-shell system X with spin S and the corresponding ions having a hole or an extra electron in the closed, open and virtual shells respectively. The six processes for forming ions with spin S±1/2 require two different definitions for canonical orbitals within each shell. These processes may be treated equivalently within a restricted CI using arbitrary non-canonical linear transforms of the ROHF orbitals optimal for the initial system. Canonical UHF orbitals also obey a variational principle for the ion energies, but they provide less appropriate estimates for actual states of the ion. Canonical spin-unrestricted Kohn-Sham orbitals with common exchange-correlation functionals suffer from all of the UHF problems and from selfinteraction error. They generally are not useful estimates for ion energies.
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